Pulsed-field gel electrophoresis was used to analyze the genomic organization of 16 bacteria belonging or related to the familyRhizobiaceae of the alpha subgroup of the classProteobacteria. The number and sizes of replicons were determined by separating nondigested DNA. Hybridization of anrrn gene probe was used to distinguish between chromosomes and plasmids. Members of the genus Agrobacterium all possess two chromosomes, and each biovar has a specific genome size. As previously demonstrated

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... r Agrobacterium tumefaciens C58, the smaller chromosomes of Agrobacterium biovar 1 andAgrobacterium rubi strains appear to be linear. The genomes of Rhizobium strains were all of similar sizes but were seen to contain either one, two, or three megareplicons. Only one chromosome was present in the member of the related genusPhyllobacterium. We found one or two chromosomes inRhodobacter and Brucella species, two chromosomes in Ochrobactrum anthropi, and one chromosome inMycoplana dimorpha and Bartonella quintana; all of these genera are related to the Rhizobiaceae. The presence of multiple chromosomes is discussed from a phylogenetic and taxonomic point of view.